I've always been a little leery of studies that show that somehow, a bigger monitor equals more productivity. Well, count me as no longer leery. I've been hacking on a 24" monitor I bought at a Christmas sale yesterday, and already I'm going along significantly faster than I was before. For one thing, I can now have both Firebug and the screen I'm interested in visible at the same time. That alone makes me twice as effective as before. Being able to do both and have the source code editor visible at the same time? Priceless.
Seriously. If you code at home and your monitor is still 19", do yourself a favor and go buy a bigger one. Or, as a cheap alternative, if your card supports it, buy a second monitor and dual-head. Whatever you do, get the capacity to have all your work visible: output, product, debugging information in one glance.
(Yes, I know, the title is tragically bad SEO, but I couldn't hold back from a Hellraiser quote.)